Context, Fixes: https://github.com/matrix-org/synapse/issues/9263
In the past to fix an issue with old Riots re-requesting threepid validation tokens, we raised a `LoginError` during UIA instead of `InteractiveAuthIncompleteError`. This is now breaking the way Tchap logs in - which isn't standard, but also isn't disallowed by the spec.
An easy fix is just to remove the 4 year old workaround.
